Expansion and contraction of outline and solid acute angles: effects of angle magnitude, type of display and the nature of the matching task

When a solid acute angle is reflected about its vertex to form a dual-angle display with a straight edge, the edge appears bent (the Bourdon illusion), in a manner which could be described as acute angle contraction. Smaller, but directionally similar, effects using outline angles have been reported by Rozvany and Day (1980). Since there is abundant evidence that single outline acute angles exhibit what can be described as acute-angle expansion, the following questions arise. First, do single solid angles induce angle contraction effects? Second, why do outline angles induce expansion effects in the single-angle display but not in the dual-angle display? Experiment 1 measured the angular function of tilt illusions induced by single outline and solid acute angles. In Experiment 2, which used dual-angle outline and solid displays, subjects set comparison lines parallel to either the upper or lower angle arm, or set a chevron to match the apparent bend in the whole edge formed by both arms. It was found that single angles show expansion when they are outline figures, contraction when they are solid, but that chevron settings in dual-angle displays cannot be predicted from parallel matches to the individual angle arms. These findings were consistent with the hypothesis that judgments in the domains of orientation, position, and collinearity reflect processing by parallel neural coding systems.